GammaShapeScale - changed the dispatch version of default_parametrization for Gamma (#526)

* Updated default_parametrization function for Gamma and GammaShapeScale without kwargs

* Update initialization_plugin_tests.jl - added tests for gamma pos args

* Update src/model/graphppl.jl

---------

Co-authored-by: Bagaev Dmitry <bvdmitri@gmail.com>

Author: skoghoern

src/model/graphppl.jl

@@ -262,9 +262,10 @@ GraphPPL.interface_aliases(::ReactiveMPGraphPPLBackend, ::Type{MvNormal}) = Grap
 
 GraphPPL.factor_alias(::ReactiveMPGraphPPLBackend, ::Type{Gamma}, ::GraphPPL.StaticInterfaces{(:α, :θ)}) = ExponentialFamily.GammaShapeScale
 GraphPPL.factor_alias(::ReactiveMPGraphPPLBackend, ::Type{Gamma}, ::GraphPPL.StaticInterfaces{(:α, :β)}) = ExponentialFamily.GammaShapeRate
-GraphPPL.default_parametrization(::ReactiveMPGraphPPLBackend, ::GraphPPL.Atomic, ::Type{Gamma}, rhs) = error(
-    "`Gamma` cannot be constructed without keyword arguments. Use `Gamma(shape = ..., rate = ...)` or `Gamma(shape = ..., scale = ...)`."
-)
+GraphPPL.default_parametrization(backend::ReactiveMPGraphPPLBackend, nodetype::GraphPPL.Atomic, factor::Type{Gamma}, rhs) = begin
+     @warn "'Gamma' and 'GammaShapeScale' without keywords are constructed with parameters (Shape, Scale)." maxlog=1
+     return GraphPPL.default_parametrization(backend, nodetype, ReactiveMP.inputinterfaces(factor), factor, rhs) 
+end
 
 GraphPPL.interface_aliases(::ReactiveMPGraphPPLBackend, ::Type{Gamma}) = GraphPPL.StaticInterfaceAliases((
     (:a, :α), (:shape, :α), (:β⁻¹, :θ), (:scale, :θ), (:θ⁻¹, :β), (:rate, :β)

test/model/initialization_plugin_tests.jl

@@ -716,3 +716,33 @@ end
     @test occursin("q(x)", repr(init))
     @test occursin("NormalMeanVariance", repr(init))
 end
+
+@testitem "gamma pos args warn and construct shape-scale" begin
+    using RxInfer
+    import GraphPPL: create_model, with_plugins
+
+    @model function gamma_only()
+        s ~ Gamma(shape = 2.0, rate = 1.0)
+    end
+
+    # Gamma without kwargs
+    init_gamma = @initialization begin
+        q(s) = Gamma(1, 1)
+    end
+    model = create_model(with_plugins(gamma_only(), GraphPPL.PluginsCollection(RxInfer.InitializationPlugin(init_gamma))))
+    node = GraphPPL.getcontext(model)[:s]
+    @test GraphPPL.hasextra(model[node], RxInfer.InitMarExtraKey)
+    @test GraphPPL.getextra(model[node], RxInfer.InitMarExtraKey) == Distributions.Gamma(1.0, 1.0)
+    @test occursin("Gamma{Float64}(α=1.0, θ=1.0)", repr(GraphPPL.getextra(model[node], RxInfer.InitMarExtraKey)))
+
+    # GammaShapeScale without kwargs
+    init_gss = @initialization begin
+        q(s) = GammaShapeScale(1, 1)
+    end
+
+    model = create_model(with_plugins(gamma_only(), GraphPPL.PluginsCollection(RxInfer.InitializationPlugin(init_gss))))
+    node = GraphPPL.getcontext(model)[:s]
+    @test GraphPPL.hasextra(model[node], RxInfer.InitMarExtraKey)
+    @test GraphPPL.getextra(model[node], RxInfer.InitMarExtraKey) == Distributions.Gamma(1.0, 1.0)
+    @test occursin("Gamma{Float64}(α=1.0, θ=1.0)", repr(GraphPPL.getextra(model[node], RxInfer.InitMarExtraKey)))
+end